One of the fundamental components of a garden, present since the earliest recorded gardens, is water. Its role in early Persian gardens, as a cooling antidote, in a shady enclosure, to the violence of the natural environment beyond - the sun, dust, heat and exposure of the desert - became transmuted in the Islamic garden tradition that developed from this into a religious symbol. The presence of canals and pools in Roman gardens also feeds into the historical tradition in the West, and water has been used in one form or another in almost every significant garden since.
But how do you like your water? Many early applications celebrated the sounds and movement of this element, enlivening otherwise controlled and formal spaces. Huge efforts were made to bring water vast distances, where necessary, and bring the force of gravity to bear in the creation of bubbling basins and rills, fountains and cascades - the very existence of these features indicated the wealth and influence of the garden's maker, and, especially in the desert zones, must have seemed a truly paradisaical relief from the world outside.
The other obvious quality that has been equally exploited is the capacity of water to reflect the sky, and whether it be a huge formal cistern as in the Islamic tradition, a meandering lake worthy of Claude in the English Landscape tradition or a simple wildlife pond, the ability to bring light and the changing moods of the sky to ground level is a powerful design tool.
Fountains can be focal points drawing the eye, rills and canals can draw the feet through the garden, formal pools with statuary can act as punctuation points in a terrace or at the end of a vista. There are few more effective ways of improving the biodiversity of a garden than adding a pool - even a relatively small tank or planter filled with water will do the job - and few more effective ways of enticing visitors to linger.
Whole gardens, particularly in Renaissance Italy, have been devoted to exploiting the countless forms that moving water can take and the equally varied sounds that it creates. It has even been used to deliberately trick visitors - seats incorporating upward pointing jets and fake trees that drench passers-by have entertained the practical jokers of the garden-making fraternity for centuries, although their guests might have had a contrary view...
Like light, water is a mutable presence in the garden - its qualities can be changed or managed to create calm, sustain interest or generate huge excitement, and for this reason alone it earns a place in every garden that can spare the space.